Solar forcing on the ice winter severity index in the western Baltic region

The Sun is the fundamental energy source of the Earth's climate and therefore its variations can contribute to natural climate variations. In the present work we study the variability of ice winter severity index in the Baltic Sea since the 15th century and its possible connection with solar activity. Based on a new method for finding and measuring amplitude–phase cross-frequency coupling in time series with a low signal/noise ratio, we suggest that the ice winter severity index in the Baltic Sea is modulated by solar activity and solar motion in several frequency bands during the last 500 years. According to our model a strong coupling between the decadal periodicity in the ice winter severity index time series and the secular periodicity of solar activity is present. We found that the ice winter severity index is strongly modulated by solar activity at the decadal periodicity. We also found that the 180 year periodicity of the Barycentre motion modulates the amplitudes of the decadal periodicity of solar activity and the ice winter severity index. This method represents a useful tool to study the solar–terrestrial relationship.

► We present a new method for assessing amplitude–phase cross-frequency coupling.
► We applied the cross-frequency coupling method to different time series.
► The ice winter severity index is strongly modulated by solar activity.